docs(project): sync docs with code and bump to 3.0.0

Prepare the 3.0.0 release:
- Bump the version in openapi.yaml and frontend/package.json to 3.0.0.
- Document the existing GET /health endpoint in openapi.yaml (served at the
  root, outside /api/v1) and refine the auth note.
- Add docs/REQUIREMENTS.md (product requirements, in English) and
  docs/ARCHITECTURE.md (system overview); remove the old Russian
  docs/Описание.md.
- Rewrite GO_PROJECT_STRUCTURE.md and FRONTEND_STRUCTURE.md to match the
  current code (dedup CLI, imagehash, real components/stores, Tailwind v4).
- Fix stale counts and references in CLAUDE.md and link the new docs from
  README.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -8,6 +8,8 @@ built SPA on one port.
 ## Documentation
 
 - [`openapi.yaml`](openapi.yaml) — full REST API specification
+- [`docs/REQUIREMENTS.md`](docs/REQUIREMENTS.md) — product requirements
+- [`docs/ARCHITECTURE.md`](docs/ARCHITECTURE.md) — system architecture overview
 - [`docs/DEPLOY.md`](docs/DEPLOY.md) — production deploy (Gitea Actions → host)
 - [`docs/GO_PROJECT_STRUCTURE.md`](docs/GO_PROJECT_STRUCTURE.md) — backend architecture
 - [`docs/FRONTEND_STRUCTURE.md`](docs/FRONTEND_STRUCTURE.md) — frontend architecture